U.S. patent application number 10/899001 was filed with the patent office on 2005-01-13 for planar lightware circuit device and manufacturing method therefor.
This patent application is currently assigned to Fujitsu Limited. Invention is credited to Itou, Michiharu, Sakurai, Yasuki, Shiotani, Takashi.
Application Number | 20050008282 10/899001 |
Document ID | / |
Family ID | 29208014 |
Filed Date | 2005-01-13 |
United States Patent
Application |
20050008282 |
Kind Code |
A1 |
Sakurai, Yasuki ; et
al. |
January 13, 2005 |
Planar lightware circuit device and manufacturing method
therefor
Abstract
A manufacturing method for a planar lightwave circuit device. A
lift-off mask layer is formed on a planar lightwave circuit
composed of cores and a cladding. The lift-off mask layer is next
exposed to light by using a mask having a plurality of first
patterns respectively corresponding to the cores and a plurality of
second patterns each formed on at least one side of each first
pattern in spaced relationship therewith. A wiring pattern material
layer is next deposited on the lift-off mask layer exposed above,
and the lift-off mask layer is next stripped off to thereby form a
plurality of real patterns respectively corresponding to the first
patterns and a plurality of dummy patterns respectively
corresponding to the second patterns, from the wiring pattern
material layer.
Inventors: |
Sakurai, Yasuki; (Kawasaki,
JP) ; Itou, Michiharu; (Kawasaki, JP) ;
Shiotani, Takashi; (Kawasaki, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700
1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
Fujitsu Limited
Kawasaki
JP
|
Family ID: |
29208014 |
Appl. No.: |
10/899001 |
Filed: |
July 27, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10899001 |
Jul 27, 2004 |
|
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|
10273091 |
Oct 18, 2002 |
|
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6787867 |
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Current U.S.
Class: |
385/14 |
Current CPC
Class: |
G02B 6/132 20130101;
G02B 2006/12176 20130101 |
Class at
Publication: |
385/014 |
International
Class: |
G02B 006/12 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 23, 2002 |
JP |
2002-120665 |
Claims
1. Canceled
2. Canceled
3. Canceled
4. Canceled
5. Canceled
6. Canceled
7. Canceled
8. Canceled
9. A manufacturing method for a planar lightwave circuit device,
comprising the steps of: depositing an undercladding layer on a
substrate; depositing a core layer on said undercladding layer;
etching said core layer to form a plurality of cores; depositing an
overcladding layer on said undercladding layer so as to cover said
cores; forming a lift-off mask layer on said overcladding layer;
patterning said lift-off mask layer so as to form a plurality of
first opening patterns respectively corresponding to said cores and
a plurality of second opening patterns each formed on at least one
side of each first opening pattern in spaced relationship
therewith; depositing a wiring pattern material layer on said
lift-off mask layer; and stripping off said lift-off mask layer
patterned to thereby form a plurality of first patterns
respectively corresponding to said cores and plurality of second
patterns each formed on at least one side of each first pattern in
spaced relationship therewith, from said wiring pattern material
layer.
10. A manufacturing method for a planar lightwave circuit device
according to claim 9, wherein said lift-off mask layer comprises a
photoresist.
11. A manufacturing method for a planar lightwave circuit device
according to claim 9, wherein said lift-off mask layer comprises a
polyimide film.
12. A manufacturing method for a planar lightwave circuit device
according to claim 9, wherein each of said first patterns and each
of said second patterns are formed of a material selected from the
group consisting of Ti, Pt, Ni, Rh, Cr, and Ta.sub.2N or an alloy
thereof.
13. A manufacturing method for a planar lightwave circuit device
according to claim 12, wherein each of said first patterns
comprises a thin film heater.
14. A manufacturing method for a planar lightwave circuit device
according to claim 9, wherein said second patterns are formed at
positions such that the volume of a portion of said lift-off mask
layer present between each first pattern and each second pattern on
one side of each first pattern is substantially equal to the volume
of another portion of said lift-off mask layer present between each
first pattern and each second pattern on the other side of each
first pattern.
15. A manufacturing method for a planar lightwave circuit device
according to claim 10, further comprising the step of baking said
photoresist at a given temperature.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a planar lightwave circuit
device such as a variable optical attenuator (VOA), optical
modulator, and optical switch, and also to a manufacturing method
therefor.
[0003] 2. Description of the Related Art
[0004] To realize a larger-scale optical transmission line system
with an increase in communication capacity, an optical device is
also desired to have a configuration of higher integration and
multiple channels. As an optical device responding to such demands,
a planar lightwave circuit device (PLC device) is known. The PLC
device can be manufactured by utilizing a semiconductor fabrication
process including film deposition and etching, and the whole
process can be performed in the condition of a wafer, which
provides superior mass productivity. Furthermore, since a device
configuration with no driving parts can be made, the PLC device is
also superior in stability.
[0005] The PLC device is manufactured in the following manner. An
undercladding layer is first deposited on an Si substrate by CVD. A
core layer is next deposited on the undercladding layer by CVD. The
core layer is next etched by photolithography and reactive ion
etching (RIE) to form a plurality of cores. An overcladding layer
is next deposited on the undercladding layer by CVD so as to embed
the cores. Each of the undercladding layer, the core layer, and the
overcladding layer is formed by a film of SiO.sub.2 doped with B,
P, or Ge so as to change a refractive index.
[0006] Since the overcladding layer is deposited above the cores,
the surface of the overcladding layer is formed with a plurality of
ridge portions respectively corresponding to the cores. These ridge
portions can be flattened under the conditions of heat treatment
(high temperature and long duration) after deposition of the
overcladding layer. However, such heat treatment causes deformation
of the cores and deviation of the core positions, causing a
degradation in optical device characteristics. Accordingly, it is
difficult to satisfy both the optical device characteristics and
the flattening of the surface of the PLC device. Further, the
flattening as by polishing makes the fabrication process
complicated. Therefore, particularly in a PLC device controlling
light by using an external signal (electrical signal), it is
desirable to adopt a technique for forming high-accuracy wiring
patterns on the uneven surface of a cladding layer as having the
ridge portions above the cores.
[0007] As an optical device having wiring patterns on the uneven
surface of a PLC with ridge portions, a variable optical attenuator
(VOA) utilizing a thermo-optic effect is known, for example. The
VOA is an optical device of such a type that thin film heaters are
formed on the surface of an overcladding at positions above the
cores, that electric powers are applied to the thin film heaters to
thereby control the phase of light with a change in refractive
index of the cores by the thermo-optic effect, and that the
attenuation is controlled by utilizing the interference of light.
As primary methods for forming wiring patterns including the thin
film heaters, dry etching, wet etching, and a lift-off process are
expected. Of these methods, both the dry etching and the wet
etching have a decisive disadvantage that the material usable for
the wiring patterns is limited.
[0008] To the contrary, the lift-off process has excellent
advantages that the wiring patterns can be formed of any material
that can be evaporated or sputtered, that is, the degree of freedom
of material selection is high, and that any special apparatus such
as an RIE apparatus is not required. The thin film heaters in the
VOA are formed by the lift-off process in the following manner. A
photoresist is first applied to the surface of a planar lightwave
circuit (PLC) composed of a cladding and cores embedded in the
cladding. The photoresist is next prebaked at a given temperature.
The photoresist is next exposed to light by using a mask, and
exposed portions of the photoresist corresponding to the cores are
next removed by development. The photoresist after the development
is postbaked at a given temperature. Thereafter, a thin film heater
material such as Ti/Pt is uniformly deposited on the photoresist,
and the photoresist is next removed by an organic solvent to
thereby form the thin film heaters on the cladding at the positions
corresponding to the cores.
[0009] In forming wiring patterns such as thin film heaters by the
lift-off process, resist shrinkage occurs due to the baking of the
photoresist or the stress during the film deposition. This resist
shrinkage strongly depends on the thickness of the resist present
between the patterns and the length of the resist present between
the patterns. In the case of forming wiring patterns such as thin
film heaters on the uneven surface of a cladding having random
ridge portions, not only the length of the resist present between
the heater patterns, but also the thickness of the resist present
between the heater patterns becomes nonuniform. Accordingly, as
compared with the case of forming wiring patterns on the flat
surface of a cladding, variations in dimensions of the patterns
become remarkable, resulting in a reduction in yield of device
chips.
[0010] This problem will now be further described with reference to
FIG. 1. A planar lightwave circuit 4 composed of a cladding 6 and
cores 8 is formed on a substrate 2. A photoresist 10 is applied to
the surface of the planar lightwave circuit 4. After baking the
photoresist 10, patterning of the photoresist 10 is performed. At
this time, the shrinkage of the photoresist 10 occurs. This resist
shrinkage is accelerated by the stress during the film deposition
to be performed later. Accordingly, while the desired dimensions in
the resist pattern are a=b=c in FIG. 1, the actual dimensions
become a<b, c because of the resist shrinkage. This is due to
the fact that the volume of each side portion of the photoresist 10
is larger than the volume of a central portion of the photoresist
10 as viewed in FIG. 1, so that the amount of shrinkage of the
photoresist 10 at each side portion becomes larger than that at the
central portion.
[0011] Particularly in the VOA, each thin film heater must be
positioned over the corresponding core symmetrically with respect
to the corresponding core, so as to efficiently supply heat to the
corresponding core. However, when the volume of a portion of the
photoresist 10 on one side of each heater pattern is different from
the volume of another portion of the photoresist 10 on the other
side of this heater pattern as shown in FIG. 2, the amounts of
resist shrinkage on the opposite sides of the heater pattern become
different from each other. As a result, the center position of the
heater pattern shown by Q in FIG. 2 is deviated from an extension P
from the center of the core 8, causing an increase and variations
in power consumption.
[0012] FIG. 3 shows the relation between resist length and heater
line width in the case that the heater is formed by using a mask
having a width of 33 .mu.m. As apparent from FIG. 3, the heater
line width increases with an increase in the resist length. This
will be due to the fact that the resist shrinkage becomes larger
with an increase in the resist length. FIG. 4 shows the relation
between resist length and variations in heater line width in the
plane of a wafer. As apparent from FIG. 4, the variations in heater
line width increase with an increase in the resist length.
SUMMARY OF THE INVENTION
[0013] It is therefore an object of the present invention to
provide a planar lightwave circuit device improved in formed
position and dimensional accuracy of a real pattern.
[0014] It is another object of the present invention to provide a
manufacturing method for a planar lightwave circuit device which
can suppress the resist shrinkage in a lift-off wiring process to
improve the forming position and dimensional accuracy of a real
pattern.
[0015] In accordance with an aspect of the present invention, there
is provided a planar lightwave circuit device including a
substrate; a planar lightwave circuit formed on the substrate, the
planar lightwave circuit having a cladding and a plurality of cores
embedded in the cladding; a plurality of first patterns formed on
the cladding so as to respectively correspond to the cores; and a
plurality of second patterns formed on the cladding so that each of
the second patterns is arranged on at least one side of each first
pattern in spaced relationship therewith.
[0016] Preferably, each first pattern includes a thin film heater
formed of a material selected from the group consisting of Ti, Pt,
Ni, Rh, Cr, and Ta.sub.2N or an alloy thereof. Preferably, each
second pattern is formed of a material selected from the group
consisting of Ti, Pt, Ni, Rh, Cr, and Ta.sub.2N or an alloy
thereof, and one of the second patterns formed at the outermost
position on the planar lightwave circuit has a width larger than
that of each of the other second patterns.
[0017] In accordance with another aspect of the present invention,
there is provided a manufacturing method for a planar lightwave
circuit device, including the steps of depositing an undercladding
layer on a substrate; depositing a core layer on the undercladding
layer; etching the core layer to form a plurality of cores;
depositing an overcladding layer on the undercladding layer so as
to cover the cores; forming a lift-off mask layer on the
overcladding layer; patterning the lift-off mask layer so as to
form a plurality of first opening patterns respectively
corresponding to the cores and a plurality of second opening
patterns each formed on at least one side of each first opening
pattern in spaced relationship therewith; depositing a wiring
pattern material layer on the lift-off mask layer; and stripping
off the lift-off mask layer patterned to thereby form a plurality
of first patterns respectively corresponding to the cores and a
plurality of second patterns each formed on at least one side of
each first pattern in spaced relationship therewith, from the
wiring pattern material layer.
[0018] Preferably, the second patterns are formed at positions such
that the volume of a portion of the lift-off mask layer present
between each first pattern and each second pattern on one side of
each first pattern is substantially equal to the volume of another
portion of the lift-off mask layer present between each first
pattern and each second pattern on the other side of each first
pattern. Preferably, each first pattern and each second pattern are
formed of a material selected from the group consisting of Ti, Pt,
Ni, Rh, Cr, and Ta.sub.2N or an alloy thereof. Preferably, the
lift-off mask layer includes a photoresist.
[0019] The above and other objects, features and advantages of the
present invention and the manner of realizing them will become more
apparent, and the invention itself will best be understood from a
study of the following description and appended claims with
reference to the attached drawings showing some preferred
embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a sectional view for illustrating a problem in the
prior art method;
[0021] FIG. 2 is a sectional view for illustrating another problem
in the prior art method;
[0022] FIG. 3 is a graph showing the relation between resist length
and heater line width;
[0023] FIG. 4 is a graph showing the relation between resist length
and variations in heater line width in the plane of a wafer;
[0024] FIG. 5 is a plan view showing the layout of VOA chips on a
wafer;
[0025] FIG. 6 is a diagram showing the layout of a two-stage MZI
type arrayed VOA chip;
[0026] FIG. 7 is an enlarged view of each VOA shown in FIG. 6;
[0027] FIG. 8 is a cross section taken along the line 8-8 in FIG.
7;
[0028] FIGS. 9A to 9H are perspective views for illustrating a
manufacturing method for a planar lightwave circuit device
according to a preferred embodiment of the present invention;
and
[0029] FIG. 10 is a sectional view for illustrating the correction
for deviation of the center position of a VOA heater by dummy
patterns.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] Referring to FIG. 5, there is shown an example of the layout
of VOA chips 14 formed in an Si wafer 12. As well known in the art,
all the processing steps including film deposition and etching are
performed in the condition of the wafer 12 to complete the VOA
chips 14. Thereafter, the VOA chips 14 formed in the wafer 12 are
separated into individual chips by cutting the wafer 12. FIG. 6
shows the layout of a two-stage MZI type arrayed variable optical
attenuator chip (VOA chip). In this layout, two Mach-Zehnder
interferometer (MZI) type VOAs 16 are connected in series to obtain
a high extinction ratio.
[0031] FIG. 7 is an enlarged view of each VOA 16 shown in FIG. 6
according to a preferred embodiment of the present invention, and
FIG. 8 is a cross section taken along the line 8-8 in FIG. 7. As
shown in FIG. 8, a planar lightwave circuit 20 is formed on an Si
substrate 18. The planar lightwave circuit 20 is composed of a
cladding 22 and cores 24 and 26 embedded in the cladding 22.
Reference numerals 28 and 30 in FIG. 7 denote 3-dB directional
couplers. A thin film heater 32 of Ti/Pt is formed on the cladding
22 at a position directly above the core 24. Similarly, a thin film
heater 34 of Ti/Pt is formed on the cladding 22 at a position
directly above the core 26.
[0032] Reference numerals 36, 38, and 40 denote dummy patterns,
which are also formed of Ti/Pt. The dummy pattern 40 is formed at
the outermost position on the chip, and the width of the dummy
pattern 40 is set larger than that of each of the other dummy
patterns 36 and 38. As will be hereinafter described, the reason
for this setting is that the amount of shrinkage of a photoresist
in formation of the thin film heaters 32 and 34 must be considered.
Ti is used as an adhering material to the cladding 22. That is, the
thin film heaters 32 and 34 and the dummy patterns 36, 38, and 40
are formed by first depositing Ti on the cladding 22 and next
depositing Pt on the Ti layer.
[0033] The thin film heaters 32 and 34 and the dummy patterns 36,
38, and 40 may be formed of a material selected from the group
consisting of Ti, Pt, Ni, Rh, Cr, and Ta.sub.2N or an alloy
thereof. The thin film heater 32 is connected to a pair of
electrodes 33, and the thin film heater 34 is connected to a pair
of electrodes 35. The electrodes 33 and 35 are formed of Ti/Au, for
example.
[0034] Signal light entered one end of the core 26 is split at a
ratio of 50:50, for example, by the directional coupler 28, and the
half of the signal light propagates along the core 24, while the
remaining half propagating along the core 26. When electric powers
are applied to the thin film heaters 32 and 34, the refractive
indices of the cores 24 and 26 are changed by a thermo-optic
effect, and the phases of the signal lights propagating along the
cores 24 and 26 are accordingly changed. With this change in the
phases of the signal lights propagating along the cores 24 and 26,
the coupling ratio of the directional coupler 30 on the output side
changes. Accordingly, by controlling the electric powers to be
applied to the thin film heaters 32 and 34, the proportion of the
second outgoing light from one of the output ports of the
directional coupler 30 to the first outgoing light from the other
output port can be controlled. That is, the power of the first
outgoing light or the second outgoing light from the directional
coupler 30 to the power of the incoming light to the directional
coupler 28 can be attenuated according to the electric powers to be
applied to the thin film heaters 32 and 34.
[0035] There will now be described a manufacturing method for a
planar lightwave circuit device according to a preferred embodiment
of the present invention with reference to FIGS. 9A to 9H. As shown
in FIG. 9A, an undercladding layer 44 is first deposited on an Si
substrate 42 by CVD. As shown in FIG. 9B, a core layer 46 is next
deposited on the undercladding layer 44 by CVD. As shown in FIG.
9C, the core layer 46 is next etched by photolithography and
reactive ion etching (RIE) to form a core 48. As shown in FIG. 9D,
an overcladding layer 50 is next deposited on the undercladding
layer 44 by CVD so as to embed the core 48. Since the core 48 is
embedded in the overcladding layer 50, the surface of the
overcladding layer 50 is formed with a ridge portion 52
corresponding to the core 48.
[0036] As shown in FIG. 9E, a photoresist 54 as a lift-off mask is
next applied to the surface of the overcladding layer 50 to such a
thickness as to cover the ridge portion 52 directly above the core
48. As the photoresist 54, a photoresist capable of forming an
overhang is preferable. For example, AZ5200NJ manufactured by
Clariant Japan K.K. may be used. The lift-off mask is not limited
to a photoresist. After applying the photoresist 54, prebaking
therefor is performed at about 90.degree. C. Thereafter, the
photoresist 54 is exposed to light by using an exposure mask having
a heater pattern and dummy patterns, thereby transferring the mask
patterns onto the photoresist 54.
[0037] As shown in FIG. 9F, the photoresist 54 after exposure is
next developed to form a resist pattern 56 left by the removal of
exposed portions of the photoresist 54 corresponding to the heater
pattern and the dummy patterns. Thereafter, postbaking for the
photoresist 54 is performed at about 120.degree. C. to complete the
resist pattern 56. The position and size of each dummy pattern are
suitably determined according to the layout of the planar lightwave
circuit. Basically, if the spacing between each dummy pattern and
the heater pattern is too small, there is a possibility that the
dummy pattern and the heater pattern both formed of a deposited
material may be short-circuited by any defects such as resist
pinholes, so that a desired refractive index change cannot be given
to the core 48.
[0038] Therefore, the spacing between each dummy pattern and the
heater pattern is preferably set to such a spacing as not to
influence the device characteristics, e.g., tens of .mu.m or more.
Preferably, each dummy pattern is formed at a position such that
the volume of a portion of the photoresist 54 between the heater
pattern and the dummy pattern on one side of the heater pattern is
substantially equal to the volume of another portion of the
photoresist 54 between the heater pattern and the dummy pattern on
the other side of the heater pattern. By forming each dummy pattern
at such a position, the amount of shrinkage of the photoresist 54
due to the baking of the photoresist 54 and the stress during the
film deposition on the resist pattern 56 can be uniformed without
adverse effects on the device characteristics, so that thin film
heaters with less variations in size can be formed at accurate
positions so as to have dimensions faithful to the mask.
[0039] As shown in FIG. 9G, a heater material 58 of Ti/Pt, for
example, is deposited on the resist pattern 56 by vacuum
evaporation or sputtering. During the deposition of the heater
material 58, the shrinkage of the photoresist 54 due to the
difference in coefficient of thermal expansion between the
photoresist 54 and the heater material 58 can be reduced because
the deposited film of the heater material 58 is cut at the
positions corresponding to the dummy patterns. Accordingly, thin
film heaters with less variations in size can be formed at accurate
positions so as to have dimensions faithful to the mask.
Thereafter, the photoresist 54 is stripped off by using an organic
solvent such as acetone or resist stripper to form a thin film
heater 60 and dummy patterns 62 and 64 as shown in FIG. 9H.
[0040] While the dummy patterns 62 and 64 are formed on the
opposite sides of the thin film heater 60, the spacing between the
thin film heater 60 and each of the dummy patterns 62 and 64 is set
to a spacing wide enough to eliminate the possibility of
short-circuit between the thin film heater 60 and each of the dummy
patterns 62 and 64, thereby preventing adverse effects of the dummy
patterns 62 and 64 on the device characteristics. Accordingly, it
is not necessary to perform any postprocessing step such as a
processing step of etching away the dummy patterns 62 and 64, but
the dummy patterns 62 and 64 may be left on the device chip. Thus,
the overall process can be simplified.
[0041] Referring back to FIG. 7, the electrodes 33 and 35 are
formed by a method basically similar to the forming method for the
thin film heaters 32 and 34. After forming the thin film heaters 32
and 34, a lift-off photoresist is applied again to the surface of
the planar lightwave circuit. The photoresist is next prebaked at a
given temperature (about 90.degree. C.) and then patterned.
Further, the photoresist patterned is postbaked at a given
temperature (about 120.degree. C.) and an electrode material of
Ti/Au is uniformly deposited on the planar lightwave circuit by
vacuum evaporation or sputtering. Thereafter, the photoresist is
removed by using an organic solvent to form the electrodes 33 and
35.
[0042] The correction for deviation of the center position of a VOA
heater by dummy patterns will now be described with reference to
FIG. 10. Reference numerals 68 denote heater patterns, and
reference numerals 70 denote dummy patterns. The dummy patterns 70
are formed at positions such that the volume of a photoresist 54a
between the heater pattern 68 and the dummy pattern 70 on the left
side as viewed in FIG. 10 is substantially equal to the volume of a
photoresist 54b between the heater patterns 68 and that the volume
of a photoresist 54c between the heater pattern 68 and the dummy
pattern 70 on the right side as viewed in FIG. 10 is substantially
equal to the volume of the photoresist 54b. By forming the dummy
patterns 70 at such positions, the amounts of shrinkage of the
photoresists 54a, 54b, and 54c can be made substantially equal to
each other. Accordingly, the heater patterns 68 can be formed at
accurate positions directly above the cores 48, respectively.
[0043] While the present invention is applied to the formation of
thin film heaters for a VOA in this preferred embodiment, the
present invention is not limited to the preferred embodiment
described above, but is similarly applicable to any other PLC
devices such as an optical modulator and an optical switch.
According to the present invention as described above, the resist
shrinkage in the lift-off wiring process can be suppressed to
thereby improve the positional accuracy of wiring patterns and
reduce variations in size of the wiring patterns.
* * * * *